Body mass index indicates how much body fat you have. Although you may be familiar with calculating your BMI as an adult, accurate BMI calculations for children take into account more variables than just height and weight, such as age and sex. BMI does not measure body fat directly; rather it is an indirect way to estimate body fat.
Calculating BMI
The first step of determining if a child is of normal weight is to accurately calculate her BMI. To do this, divide her weight in pounds by her height in inches, squared. Then, multiply this value by 703. For instance, if a girl is 70 lbs. and 48 inches tall, first square her height, which gives you 2304 inches. Divide 70 lbs. by this value, and then multiply by 703 to get a BMI value of 21.4.
Percentiles
Unlike adults, a children's BMI value is plotted on the Centers for Disease Control and Prevention BMI-for-age growth chart, of which there is one for boys and one for girls, to get a percentile value. Percentiles indicate a child's BMI relative to other children of the same sex and age. Children between the fifth and 85th percentile are in a healthy weight range. Those between the 85th and 95th percentile are overweight, and children over the 95th percentile are obese. If your child is in less than the fifth percentile, he is underweight.
Weight Concerns
You should talk to your doctor if your child falls in a percentile identifying her as overweight or obese. Because BMI does not take into account differences in muscularity, having a larger-than-average body frame or varied growth patterns, your doctor can help determine if your child's weight is actually a concern. Your doctor can take into account your child's individual history of growth and development, as well as your family's history, and determine if your child's weight is in an unhealthy range.
Considerations
If your child is under age 3, you need to measure her while she is lying down to get an accurate measurement. You can measure children aged 3 and older while standing. According to the University of Maryland Medical Center, doctors recommend you start calculating your child's BMI beginning at 2 years of age to screen for weight issues as BMI is an important way to identify and prevent obesity. It is important to remember that growth charts are not an accurate predictor of your child's future full-grown height, but talk to your doctor if you have any concerns.
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